The overall goal of this application is to provide new insight into changes in the regulation of brain vascular tone following brain injury. Specifically, the proposed studies will investigate the mechanisms that subserve the delayed dilatation of brain arterioles that occurs after transient exposure to the acute vasodilators bradykinin or arachidonic acid. In brain, both bradykinin and arachidonic acid produce acute, reversible vasodilatation mediated by reactive oxygen species, most likely hydrogen peroxide. Reactive oxygen species have been shown in isolated tissues and cells to cause expression of cyclooxygenase-2 (COX-2), an inducible isoform of cyclooxygenase. We hypothesize that transient exposure of brain to bradykinin or arachidonic acid produces a delayed vasodilatation that occurs hours later. We also hypothesize that this delayed vasodilatation is mediated almost exclusively by COX-2, whose expression is upregulated by reactive oxygen species. The studies proposed in this application will use a cranial window model in anesthetized rats. Cranial windows offer the unique ability to study the cerebral circulation where it remains integrated brain tissue. Initial studies will undertake a systematic exploration of the concentration- and time-dependence of the delayed dilatation produced by acute exposure to bradykinin or arachidonic acid. Subsequent studies will use pharmacologic antagonists to establish the role of COX-2 and inducible NO-synthase in the delayed vasodilatation. Immunohistochemical studies will verify the expression of COX-2 protein and also identify the cell type expressing COX-2 by use of double labeling with confocal microscopy. The final series of projects will investigate the role of reactive oxygen species in bradykinin- and arachidonic acid-mediated delayed expression of COX-2. The specific role of hydrogen peroxide will be investigated, as well as the ability of hydrogen peroxide alone to induce delayed, COX-2 dependent dilatation. These studies will introduce an important new concept in the cerebral circulation - that acute dilators such as bradykinin and arachidonic acid can cause changes in gene expression that underlie delayed vascular effects.